The present invention relates generally to photolithography, and particularly to an exposure system to control variations in linewidth in an image that is printed and used in the manufacture of semiconductor devices.
In semiconductor manufacturing, the image of a reticle or mask is projected onto a photosensitive substrate or wafer. As semiconductor devices become ever smaller, the feature size of the images printed on the semiconductor device also become smaller. Correctly imaging and printing these small feature sizes onto a photosensitive substrate becomes increasingly difficult as the feature size is reduced. As the feature sizes approach the fraction of exposure wavelength, correct imaging is often difficult to obtain. There are many variables that determine the image quality and correct printing of a pattern on a reticle. The lines on a reticle to be reproduced may vary as a function of the feature size, type, and location in the field. The image may also vary as a function of the orientation or direction of the feature on the mask or reticle being imaged. There have been many attempts to improve the imaging characteristics of a photolithographic device to improve image quality and provide consistent printing. One such photolithographic system is disclosed in U.S. Pat. No. 5,383,000 entitled xe2x80x9cPartial Coherence Varier For Microlithographic Systemsxe2x80x9d issuing to Michaloski et al on Jan. 17, 1995, which is herein incorporated by reference. Therein disclosed is a microlithographic system utilizing an adjustable profiler that is actually movable along the optical axis in the illumination path for imposing a predetermined angular profile of the illumination. Another device for improving imaging in a photolithographic system is disclosed in U.S. Pat. No. 6,013,401 entitled xe2x80x9cMethod of Controlling Illumination Field To Reduce Linewidth Variationxe2x80x9d issuing to McCullough et al on Jan. 11, 2000, which is herein incorporated by reference. Therein disclosed is an dynamically adjustable slit for controlling the exposure dose at different spatial locations during a scanning exposure of a reticle. The adjustments in exposure are made in a direction perpendicular to the direction of scan of the illumination field. Another photolithographic system is disclosed in U.S. patent application Ser. No. 09/232,756 entitled xe2x80x9cDose Control For Correcting Linewidth Variation In The Scan Directionxe2x80x9d filed Jan. 15, 1999, by McCullough, which is herein incorporated by reference. Therein disclosed is a device for varying the exposure dose as a function of distance in a scan direction for reducing linewidth variation. While these photolithographic systems and exposure devices and methods have improved image quality and the printing of features upon a photosensitive substrate, there is a need for yet further improvement, especially as the feature size is reduced and the need for better image quality and correct reproduction of the image. on a photosensitive substrate.
The present invention is directed to a method and apparatus for correcting for printed feature size variations over a field by selectively adjusting the dose and partial coherence in a scanning photolithographic device. A scanning photolithographic device or tool comprises an illumination source forming an illumination field that is scanned, projecting the image of a mask or reticle onto a photosensitive substrate. The illumination field is adjusted to vary the exposure dose at predetermined spatial locations and an adjustable array optical element selectively varies at different spatial locations the numerical aperture of the illumination, thereby varying the partial coherence of the system.
Accordingly, it is an object of the present invention to improve the printing of features on a photosensitive substrate.
It is a further object of the present invention to correct for feature size variations resulting from the mask error factor.
It is yet a further object of the present invention to correct for reticle linewidth variations.
It is an advantage of the present invention that it is easily modifiable to accommodate different features to be printed.
It is a further advantage of the present invention that it reduces linewidth variances in printed features.
It is yet a further advantage of the present invention that it reduces horizontal and vertical bias in printed features.
It is a feature of the present invention that an adjustable slit is used to vary the exposure dose.
It is another feature of the present invention that an array optical element is used to vary the illumination numerical aperture and resulting partial coherence at different locations in the illumination field to minimize reticle feature variation induced horizontal/vertical bias.
These and other objects, advantages, and features will become readily apparent in view of the following more detailed description.